1. Field of the Invention
The present invention relates to an apparatus for controlling a continuously variable transmission and to a method thereof. More specifically, the invention relates to technology for improving characteristics in the speed change control in response to the operation of the accelerator.
2. Related Art of the Invention
In the conventional continuously variable transmissions, it is accepted practice to set a basic speed change ratio based upon the opening degree of the throttle valve (opening degree of the accelerator) and the vehicle speed and to change the actual speed change ratio based upon a predetermined rate of changing the speed, so that the actual speed change ratio gradually approaches the basic speed change ratio (see Japanese Unexamined Patent Publication No. 62-4644).
In the continuously variable transmission as described above, when the opening degree of the throttle valve is changed causing the basic speed change ratio to greatly change stepwisely, then the actual speed change ratio starts approaching the basic speed change ratio that has changed at a predetermined rate of changing the speed. In particular, at the time of up-shifting in which the throttle valve (accelerator) is returned to the fully closed state, when the rate of changing the speed is too large, the driver feels during the high-speed cruising that the car is accelerating due to inertial torque. During the low-speed running, on the other hand, the driver feels that the car has suddenly lost the power. To alleviate such incompatible feeling, the actual speed change ratio must be brought gradually to the basic speed change ratio at a relatively slow rate of changing the speed.
Therefore, when the basic speed change ratio stepwisely changes toward the high-speed side as a result of returning the throttle valve to the fully closed state, the actual speed change ratio gradually approaches, at a relatively slow rate, the basic speed change ratio of the high-speed side that corresponds to the state where the throttle valve is fully closed. In a state where the actual speed change ratio is approaching the basic speed change ratio, when the throttle valve is opened (accelerator is trod) but when a change in the basic speed change ratio accompanying the opening operation of the throttle valve does not traverse the practical speed change ratio, the actual speed change ratio still remains on the side lower than the basic speed change ratio, and the speed change ratio continues to change toward the high-speed side (up-shifting).
Therefore, despite the driver attempts to accelerate the car by shifting to the low-speed side (down-shifting) by pressing the accelerator, the speed (up-shifting) continues to change toward the high-speed side independently of the intention of the driver who then feels incompatibility in driving.
Similarly, when the opening degree of the throttle valve is decreased as a result of returning back the accelerator while the basic speed change ratio is stepwisely changing toward the low-speed side in response to the press of the accelerator and while the actual speed change ratio is gradually approaching the basic speed change ratio of the low-speed side at a predetermined rate of changing the speed, the change of speed to the low-speed side (down-shifting) is continued despite the driver intends to change the speed (up-shifting) toward the high-speed side. The driver therefore feels incompatibility in driving.